This invention relates to a nuclear reactor, and more particularly, to a lower part structure of a core suitable for a boiling water reactor.
Japanese Patent Laid-open application No.2-68195 discloses that, in a nuclear reactor of the type, in which the reinforcing structures, such as beams, for the core support plates are provided to a control rod guide tube at a position near a coolant entrance, a pressure loss at an entrance orifice can be reduced by installing coolant entrances of the control rod guide tubes and fuel support pieces near the lower end of the reinforcing structures of the core support plate.
Atomic Energy Industry, volume 38, No. 11, p. 28 discloses the following.
A coolant guide tube is provided over the lower plenums and the entrances of the fuel assemblies, so that flow stability of the two-phase flow in the fuel assemblies is improved by the inertia of the high-speed coolant which flows inside of the control rod guide tube. In this prior fuel assembly construction, the entrance of the coolant guide tube is formed at a position lower than the lower plenum.
In the conventional technique described in the afore-mentioned Japanese Patent Laid-open application, it is possible to reduce the pressure loss of the coolant flow between the entrance orifice and the outside of the control rod guide tube. However, the pressure loss between the entrance orifice and the coolant entrance of the fuel assembly cannot be reduced, while maintaining the flow stability of the two-phase flow in the fuel assemblies. For this reason, when a pressure loss in the fuel assemblies becomes larger than the pressure loss between the entrance orifice and the coolant entrance of the fuel assemblies, the flow stability of the two-phase flow in the fuel assemblies decreases.
Because the entrance orifice is located at a position lower than that of the conventional nuclear reactor, the ascending distance of the coolant is shorter than that in the conventional arrangement so that the coolant is not mixed sufficiently. The coolant is discharged from a recirculation internal pump in spaces between the control rod guide tubes.
In the conventional technique described in the Atomic Energy Industry publication, the tube length of the coolant guide tube is made larger, or the tube diameter of the coolant guide tube is made smaller, so as to improve the flow stability of the two-phase flow in the fuel assemblies. If the tube length of the coolant guide tube is made larger, the maximum value of the length is restricted to the height of the lower structure of the core. If the tube diameter of the coolant guide tube is made smaller, it is impossible to sufficiently reduce pressure loss between the entrance orifice of the lower plenum and the coolant entrance of the fuel assemblies.
Therefore, in order to improve the flow stability of the two-phase flow in the fuel assemblies and to sufficiently reduce the pressure loss between the lower plenums and the coolant entrance of the fuel asaemblies, the length of the coolant guide tube is made larger. Because the entrance of the coolant guide tube is located at the lower part of the lower plenum, the flow of coolant discharged from the recirculation internal pump does not mix sufficiently.